This invention relates to a method for preparing granulated alkaline earth metal carbonate, to granulated alkaline earth metal carbonate produced by the method of the invention, and to use of such granulated alkaline earth metal carbonate in the glass industry.
Alkaline earth metal carbonate, particularly barium carbonate and strontium carbonate, are used in the glass industry, for example in the manufacture of faceplates for television receivers. In this way X-rays from cathode ray tubes can be absorbed.
One method for producing very pure alkaline earth metal carbonates is the precipitation of alkaline earth metal hydroxides or alkaline earth metal sulfides with carbon dioxide or carbonates. In this process very fine powders are formed, which are difficult to handle. In the past, such powders have been granulated by sintering at about 800.degree. C. in rotary kilns which are lined with inorganic refractory materials, e.g., with amorphous silicon dioxide, aluminosilicates or fire clay. In such kiln granulation processes, it is absolutely impossible to prevent detritus of refractory material from contaminating the granules. Due to these impurities, a considerable proportion of the picture tubes produced have to be considered unusable and must be remelted.
Adams et al., U.S. Pat. No. 4,888,161 describes a method for decreasing the contamination of the carbonate by detritus of refractory material attributable to the high sintering temperature of 800.degree. C. In that process, a suspension of the carbonate in water initially is prepared, and an amount of a deflocculant sufficient to deflocculate the carbonate is added. Preferred deflocculants are alkali metal salts or ammonium salts of acrylic acid or methacrylic acid or of phosphoric acid or polyphosphoric acid. The fluid suspension is then dried in a spray dryer, and the resulting granular product is heated to 600.degree. to 700.degree. C. The type of heating used is described as not critical.
There remains a need for an improved method of producing granulated, high purity alkaline earth metal carbonate.